The present invention relates to the transmission of data in the context of pipeline systems, for example, surface, sub-sea or downhole pipelines.
U.S. Pat. No. 5,130,706 discloses an apparatus for transmitting data from a downhole location in a borehole to the surface. A downhole power source is used to apply a signal to an earth-drillstring system at the downhole location. Data is transmitted from the downhole location along the drillstring and is received at the surface.
Systems in which the power requirement for the transmission of data is met from the data transmission location have disadvantages. A large amount of power can be required for transmitting data over the distances typically associated with pipeline systems. Local power supplies, for example batteries, have disadvantages. These disadvantages are more pronounced at inaccessible locations and in particular at downhole locations. The batteries may be xe2x80x98one-shotxe2x80x99 batteries or rechargeable batteries. These have limited life and replacing or recharging can be slow, expensive and wasteful of energy. Further the temperature in the downhole environment can be high and this can reduce life further, cause high self discharge and can cause electrolyte boiling problems.
U.S. Pat. No. 4,322,728 discloses the idea of using a surface based power source to extract data from a downhole location. A current pulse is sent along a wireline to a downhole transducer assembly where the resistance of each transducer is indicative of a quantity to be measured. The resistance level can be detected at the wellhead by monitoring the voltage produced by the current pulse and hence the quantity of interest can be determined. However, the system of U.S. Pat. No. 4,322,728 makes use of a wireline along which the current pulse travels. The use of separate wirelines has disadvantages in terms of expense and susceptibility to damage both during and after installation.
It is an object of the present invention to provide a data transmission system which alleviates at least some of the problems of the prior art.
According to a first aspect of the present invention there is provided a data transmission system for use with a pipeline system having at least one electrically conductive pipe arranged for carrying a fluid comprising,
reference signal generating means for applying a reference signal to a signal circuit,
effective impedance varying means for varying the effective impedance of the signal circuit in dependence on data to be transmitted, and
monitoring means for monitoring changes in the reference signal caused by varying the effective impedance of the signal circuit to thereby extract data, wherein the reference signal generating means is arranged for applying a signal to a pipe in the pipeline system whereby, in use, the signal circuit comprises said pipe.
According to a second aspect of the present invention there is provided a method of data transmission for use with a pipeline system having at least one electrically conductive pipe arranged for carrying a fluid comprising the steps of,
generating and applying a reference signal to a signal circuit,
varying the effective impedance of the signal circuit in dependence on data to be transmitted, and
monitoring changes in the reference signal caused by varying the effective impedance of the signal circuit to thereby extract data, including the steps of using a pipe in the pipeline system as part of the signal circuit and applying the reference signal to said pipe.
This arrangement has the advantage that the high power supply requirement for data transmission can be removed from the location from which data is to be transmitted whilst a separate cable or wireline is not required.
The reference signal generating means and the varying means may be disposed at distinct locations. Typically the power requirements of the reference signal generating means will be high whereas the power requirements of the varying means will be relatively low and so can be more easily met from a battery. The monitoring means and varying means are typically disposed at distinct locations.
The data transmission system may comprise the signal circuit. The signal circuit can comprise a signal channel along which data is transmitted and a return conduction path. The signal channel can comprise the conductive pipe to which the reference signal generating means is connected. The signal circuit can comprise a return conduction path via earth.
In use, one terminal of the reference signal generating means can be connected to the conductive pipe and the other to earth via an earth connection point. Preferably the reference signal generating means comprises a constant current source. This has the advantage that the monitoring means can be decoupled from any imperfections in the earth connection point used to complete the signal circuit.
The monitoring means may be arranged to monitor changes in the potential difference, due to the reference signal, between a terminal of the current source and earth. The monitoring means may be connected across the terminals of the current source. In this case the earth connection used for monitoring changes in potential difference is the same as that used to complete the signal circuit through earth. This has disadvantages because the earth connection point will not be true earth and its potential difference relative to earth will vary as the effective impedance of the signal circuit is varied.
Preferably a separate reference earthing point is provided and the monitoring means is arranged to monitor the potential difference between an output of the current source and the reference earthing point. This has advantages because the signal detected by the monitoring means is not then effected by the changing potential at the earth connection point and consequently noise is reduced. The reference earthing point can be a remote earth.
The monitoring means may comprise a voltage measuring means. One terminal of the voltage measuring means may be connected to the terminal of the current source connected to the conductive pipe and the other terminal of the voltage measuring means may be connected to the reference earthing point.
The current source can be arranged to generate a constant dc current signal.
The signal channel may comprise an isolation joint which electrically isolates adjacent sections of the signal circuit from one another. The isolation joint may be provided in the conductive pipe to electrically isolate one section of the pipe from an adjacent section.
The transmission means may be arranged for use in a well having downhole structure. The pipe to which the reference signal generating means is connected in use can comprise the downhole structure. The advantages of the system are particularly useful in downhole situations because the large power requirement for transmitting data from downhole to the surface can be met from the surface. This is done by locating the reference signal generating at the wellhead and the impedance varying means downhole.
Preferably the downhole structure is treated as a single conducting channel. The downhole structure can comprise a production string and a casing. Preferably the production string and the casing are electrically connected to form the single conducting channel.
It has been found that a system which uses the production string as a signal line and the casing as the return is unsatisfactory in circumstances where the space between the production string and the casing is filled with a high density brine solution for pressure equalisation purposes. The brine solution has a relatively high electrical conductivity. This means that a system relying on the production string and casing to form a circuit is susceptible to extremely high losses due to the conduction path directly from the production string to the casing through the brine solution. Using the downhole structure as a single conducting channel avoids the problems which arise due to the conduction path between the production string and casing which exists when the intervening space is filled with a conductive medium.
The signal circuit can comprise the downhole structure and a return conduction path via earth.
Typically when the downhole structure is disposed in a well, the production string will be positioned so as to have an encased portion disposed within the casing and an exposed portion projecting beyond the casing. Preferably at least part of the exposed portion acts as a distributed earth.
An isolation joint may be provided in the production string. Preferably the isolation joint is provided in the exposed portion of the production string. Preferably a portion of the downhole structure on one side of the isolation joint acts as a distributed earth.
The system is suitable for use in both producing and abandoned wells. The downhole structure may comprise a monitoring section which is arranged to be kept in an abandoned well. The monitoring section can comprise an isolation joint.
In some circumstances a well may include lateral developments in addition to the main bore. The structure of the laterals may or may not be electrically connected to the main bore. If the structures are not electrically connected to the main bore a short range electrical link may be provided. The link may be an inductive link between the structure of the main bore and that of the lateral.
The effective impedance varying means may comprise an isolation joint. The effective impedance varying means may comprise switch means switchable between two states, the effective impedance of the signal circuit when the switch means is in the first state being greater than the effective impedance of the signal circuit when the switch means is in the second state. The switch means may be provided across the isolation joint. The switch means may be arranged to short across the isolation joint when in the second state. The switch means may be a simple switch, the two states corresponding to open circuit and short circuit. The switch means may comprise charge storage means. Preferably, the charge storage means is operable to change the effective impedance of the signal circuit in dependence on the state of the switch means.
The switch means can be arranged so that, in use, the charge storage means is charged when the switch means is in one state and is discharged when the switch means is in the other state.
The switch means can be arranged so that, in use, the charge storage means generates a potential difference across the isolation joint. This potential difference can be such that it tends to oppose the reference signal current when the switching means is in one state and/or such that it tends to increase the reference signal current when the switching means is in the other state.
The charge storage means may be, for example, a capacitor or an electric cell.
Preferably the effective impedance varying means is an effective impedance modulation means for modulating the reference signal to encode data to be transmitted and the monitoring means is arranged for decoding data transmitted. This allows more complex data to be transmitted using a single varying means and has the advantage that a large variety of different signals can be sent using only two signal levels. Data can be transmitted as a switched digital signal. Thus to extract data it is only necessary to be able to detect two differing signal levels rather than it being necessary to measure the magnitude of the signals. The monitoring means may comprise decoding means for decoding the data transmitted. The monitoring means may comprise a CPU.
Control means can be provided for acquiring and processing the data to be transmitted and controlling the effective impedance modulating means. The control means can be arranged to accept and process data output by sensors disposed in a desired location. The control means can be arranged to produce an encoded signal representing the data to be transmitted. The control means can be arranged to switch the switch means between its two states in order to modulate the effective impedance of the signal circuit in accordance with the encoded signal representing the data to be transmitted. The control means can comprise a microprocessor.
In developments of the invention the downhole structure or signal channel is provided with a plurality of isolation joints. A plurality of effective impedance modulating means can be provided, each of which may comprise a respective isolation joint.
According to a third aspect of the present invention there is provided a data transmission system comprising,
signal generating means for generating and applying to a signal circuit a signal which represents data to be transmitted, wherein the signal generating means comprises a reference signal generating means and modulation means for modulating the reference signal to thereby encode the data to be transmitted, the modulation means comprising effective impedance modulating means for modulating the effective impedance of the signal circuit, and the effective impedance modulating means comprising switch means which is switchable between two states, the effective impedance of the signal circuit when the switch means is in the first state being greater than the effective impedance of the signal circuit when the switch means is in the second state, wherein the switch means comprises charge storage means arranged to enhance the difference in effective impedance between the first and second states.
Monitoring means may be provided for monitoring changes in the reference signal caused by the modulating means.
According to a fourth aspect of the present invention there is provided a pipeline system comprising a conductive pipe for carrying fluid and a data transmission system as defined in more detail above.
According to a fifth aspect of the present invention there is provided a well comprising downhole structure including a conductive pipe for carrying fluid and a data transmission system as defined in more detail above.
Many of the preferred features introduced in relation to the first and second aspects of the invention are equally applicable as preferred features of the third, fourth and fifth aspects of the invention.